A primary step in the fabrication of semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapour precursors. One known technique for depositing a thin film on a substrate is chemical vapour deposition (CVD). In this technique, process gases are supplied to an evacuated process chamber housing the substrate and react to form a thin film over the surface of the substrate.
A CVD process used to deposit an aluminium layer on a substrate is MOCVD (metal organic chemical vapour deposition), in which an organoaluminium precursor entrained within a carrier gas, such as nitrogen or argon, is supplied to the process chamber. Hydrogen is also supplied to the process chamber for reducing the precursor. The process chamber is evacuated, and heated to a deposition temperature, generally less than 500° C., at which the precursor decomposes and aluminium is deposited on to the substrate.
Physical vapour deposition (PVD) is a vaporisation coating technique involving the transfer of material on an atomic level. One example of a PVD technique used in semiconductor manufacture is sputter coating, in which atoms in a solid target material located within an evacuated process chamber are ejected into the gas phase due to the bombardment of the material with energetic ions. These atoms are deposited on a substrate located within the process chamber to form a thin film on the substrate.
The presence of contaminants in the residual gas of the process chamber can be detrimental to the quality of the film or layer formed on the substrate. The dominant residual gas is usually water but may also be oxygen or hydrogen. Impurities within the generated layer can result in one or more of low density, low stress film, intrinsic stress in the film, increased electrical resistivity of the film and a reduction in the positive temperature coefficient of the resistance of the film. Hydrogen, being a light gas, can be particularly intrusive and can lead to hydrogen embrittlement of the generated layer. In PVD processes the avoidance of contamination by hydrogen is, therefore, of particular importance.
In such deposition processes, the residence time of the deposition gases in the process chamber is relatively short, and so only a small proportion of the gas supplied to the chamber is consumed during processing. Consequently, much of the gas supplied to the process chamber is exhausted from the chamber with the by-products from the deposition process, and conveyed by a conduit to a vacuum pump used to evacuate the process chamber.
A process tool usually comprises a plurality of process chambers, in which similar or different processes may be conducted at any given time. For example, in addition to one or more deposition chambers, the process tool may comprise one or more etch chambers within which features are etched within the substrate and/or within the thin film deposited on the substrate. Consequently, the waste stream from one chamber of the process tool can be incompatible with a process being undertaken in another chamber of the process tool. In view of this, the pumping arrangement used to evacuate the chambers generally comprises a secondary pump for each process chamber, as illustrated in FIG. 1. FIG. 1 illustrates first and second process chambers 2, 12 each being evacuated by a respective turbomolecular vacuum pump 4, 14. Each turbomolecular vacuum pump is backed by a respective primary pump 6, 16 in order to retain separation of the waste streams exhausted from the process chambers 2, 12.
It is desirable to reduce the complexity, footprint and power requirements of the overall pumping arrangement whilst improving reliability and costs associated with the pumping arrangement. It is, therefore, preferable to provide a single primary pump to back a number of separate secondary pumps, a configuration often implemented in pumping arrangements in which contamination is not an issue. An example of this configuration is illustrated in FIG. 2, wherein each turbomolecular vacuum pump 4,14 has a respective exhaust conduit 8, 18 connected to its outlet. The exhaust conduits 8, 18 merge to form a common exhaust conduit which is connected to an inlet of the primary vacuum pump 10.
Unfortunately, if there is an incompatibility between the components of one of the waste streams, say that being exhausted by secondary pump 14, and the process being undertaken in the other chamber 2, contamination can be an issue. Three significant examples of contamination in the vacuum field are:                hydrogen, even tiny quantities of which can cause embrittlement of a sputtered film; helium, the presence of which impacts heat transfer characteristics of the environment which affect the bulk properties of the film; and water, which can be significantly detrimental as it can react with precursor materials in the process chamber.        
Contamination becomes particularly noticeable when the contaminant material is a light gas, namely a gas having a low relative molecular mass, such as hydrogen. This light gas may originate in chamber 12 and be conveyed via turbomolecular vacuum pump 14 into exhaust conduit 18. Due to the connection between the exhaust conduits 8, 18, this light gas may migrate backwards from the exhaust conduit 8 through the turbomolecular vacuum pump 4 and into chamber 2.
The partial pressure of the light gas is likely to be lower in chamber 2 than in exhaust conduit 8 due to the compression by the turbomolecular vacuum pump 4 but in some processes even trace quantities of contamination are undesirable. In these circumstances a pumping arrangement using a common primary pump 10 is inappropriate.
Most turbomolecular vacuum pumps are optimised for pumping heavier gases, rather than for light gases. Whereas a typical compression ratio for a heavier gas, such as nitrogen or argon, may exceed 1×108, the compression ratio for hydrogen is likely to be between 1×103 and 1×105 and for helium the compression ratio is likely to be in the region of 1×105 to 1×107.
Increasing the compression of a turbomolecular vacuum pump by adding additional pumping stages to the pumping mechanism would further increase the compression ratios for the light gases and therefore reduce the backward migration of a contaminant component from an outlet to an inlet of the turbomolecular vacuum pump. However, introduction of additional pumping stages would increase the cost and size of the pump.
It is an aim of the present invention to address the issue of backward migration of components of a waste stream through a vacuum pump to enable a common primary pump or common ducting to be implemented.